Computers historically have booted from direct attached storage (DAS). With the advent of storage array networks (SANs) it is now possible to boot physical computers from disks in a SAN. With the advent of Virtual Machines it is possible to boot multiple virtual computer systems on a single physical computer from virtual disk images located on either on SAN or DAS storage. Furthermore, virtual machines may boot directly from ‘pass thru’ disks located on either SAN or DAS storage.
Some iSCSI target implementations use virtual disk images as the base storage for the SAN disks. Consequently, it is possible to have a physical machine booting from a directly attached disk. It is possible to have a virtual machine booting from a virtual disk image located on a directly attached disk. It is possible to have a physical machine boot from an iSCSI disk that is actually a virtual disk image on an iSCSI target. It is also possible to have a virtual machine boot directly from a physical ‘pass thru’ disk.
The configuration and management of these various boot images and methods is complex and the choice of what method to use may depend on the performance needs of the computer system.